In the exposure step of a semiconductor manufacturing process, a circuit pattern is formed on a reticle (photomask) is projected and exposed onto a wafer applied with a resist material to form a latent image pattern on the resist material. The latent image pattern is developed thereafter to form a resist pattern for etching, ion implantation, and the like.
If a particle, or the like, is present on the reticle, the particle, together with the pattern, is transferred onto the wafer to cause an error. In order to prevent this, a pattern protective member (including, e.g., a film-like synthetic resin member or a plate-like member made of silica glass, or the like), called a pellicle, is attached to the reticle.
The pellicle is arranged at a position offset from the pattern surface of the reticle by a predetermined distance and supported by a pellicle support frame. When the pellicle is used, a particle attaches to a pellicle surface offset from the pattern surface of the reticle by the predetermined distance. Hence, the particle does not directly form an image on the wafer surface during exposure, but appears as variations in illuminance of the illumination light to decrease errors caused by the particle.
FIG. 1 is a schematic view showing the structure of a pellicle. A pellicle 24 is adhered to the pattern surface side of a reticle 23 with an adhesive, or the like, through a reticle support frame 25. The pellicle 24 is formed of the reticle support frame 25 arranged to surround the circuit pattern on the reticle, and a pellicle film (or pellicle plate) 26 is adhered to one end face of the reticle support frame 25. The pellicle film 26 has high exposure light transmittance. If the space (to be referred to hereinafter as a pellicle space), surrounded by the pellicle 24 and reticle 23, is set in a completely sealed state, the pellicle film 26 may inflate or deflate due to the atmospheric pressure difference between the inside and outside of the pellicle space, which is inconvenient. To prevent this, vent holes 27 are formed in the reticle support frame 25 so a pressure difference does not occur between the inside and outside of the pellicle space. Also, dustproof filters (not shown) are provided to prevent an external particle from entering the pellicle space through the vent holes 27. In the manufacturing process of a semiconductor device, such as an LSI or VLSI, formed of very small patterns, a reduction projection exposure apparatus is used, which reduces and projects a circuit pattern formed on a reticle onto a wafer applied with a resist agent to form a latent image pattern on the resist agent. As the integration density of the semiconductor device increases, a further decrease in the feature size of the circuit pattern is required to lead a need for development of the resist process. Simultaneously, the exposure apparatus is required to have a thinner exposure beam width.
Methods of improving the resolution performance of the exposure apparatus include a method of further decreasing an exposure wavelength and a method of increasing the numerical aperture (NA) of a projection optical system. To decrease the exposure wavelength, a KrF excimer laser with an oscillation wavelength of 365-nm i-line to around 248 nm, and an ArF excimer laser with an oscillation wavelength around 193 nm, are used. Recently, exposure using a fluorine (F2) excimer laser with an oscillation wavelength around 157 nm has also been developed.
When an ArF excimer laser with a wavelength around far ultraviolet rays, particularly, 193 nm, and a fluorine (F2) excimer laser with an oscillation wavelength around 157 nm are to be used, a problem arises in that a plurality of oxygen (O2) absorption bands are present around their wavelength bands. For this reason, the optical path of an exposure optical system in a projection exposure apparatus, which uses the ArF excimer laser with a wavelength around far ultraviolet rays, particularly, 193 nm, the fluorine (F2) excimer laser with a wavelength around 157 nm, or the like, as a light source, is purged with an inert gas, such as nitrogen. This suppresses the oxygen concentration in the optical path to a low level, on the several ppm order or less. Similarly, moisture (H2O) must also be suppressed to a low level, on the several ppm order or less.
In order to ensure the transmittance and stability of the fluorine (F2) excimer laser beam in this manner, a reticle stage including a projection lens end face and length measurement interference optical system, and an entire wafer stage, must be arranged in a hermetic chamber, and the interior of the hermetic chamber must be entirely purged with a high-purity inert gas. Also, in order to load and unload a wafer or reticle in and from the hermetic chamber with the inert gas concentration in the hermetic chamber being kept at a constant level, a load-lock chamber is arranged adjacent to the hermetic chamber. Regarding the pellicle space, the illuminance there may be undesirably similarly decreased by light absorption. To prevent this, when unloading a reticle, the pellicle space in the load-lock chamber, or the like, must be purged with an inert gas.
FIG. 2 is a schematic view showing an example of a semiconductor exposure apparatus, which uses a fluorine (F2) excimer laser as a light source and has a load-lock mechanism.
Referring to FIG. 2, a reticle on which a pattern is drawn is loaded on a reticle stage 1. The pattern on the reticle is transferred onto a wafer in an exposure unit 2. The exposure unit 2 includes a projection optical system, which projects the reticle pattern onto the wafer, an illumination optical system, which illuminates the reticle, and the like. Illumination light from a fluorine (F2) excimer laser light source (not shown) is guided to the exposure unit 2 through a guide optical system.
The reticle stage 1 is covered with a housing 8 and purged with an inert gas. The entire exposure apparatus is covered with an environment chamber 3. Air controlled to a predetermined temperature circulates in the environment chamber 3 to keep the internal temperature of the environment chamber 3 constant. Clean air, which is temperature-controlled by an air-conditioner 4, is supplied to the environment chamber 3. The air-conditioner 4 also has a function of adjusting a predetermined portion, such as an optical system, to an inert gas atmosphere.
The housing 8, which covers the reticle stage 1, is connected to a reticle load-lock 13, which is used when loading and unloading the reticle in and from the housing 8. A reticle hand 15 loads and unloads the reticle and transports the reticle in the housing 8. A reticle storage 18, which stores a plurality of reticles, is arranged in the housing 8. A particle inspection unit 19, which measures and counts the size and number of particles, such as dust attaching to the reticle surface or pellicle surface, is arranged in the housing 8.
An SMIF (Standard Mechanical InterFace) pod 14, which stores the reticle, and a reticle relay hand 16, which transports the reticle between the SMIF pod 14 and the load-lock 13, are arranged in the environment chamber 3. After the reticle is loaded in the load-lock 13, the interior of the load-lock 13 is purged with the inert gas to set it to an inert gas atmosphere identical to that in the housing 8. Then, the reticle hand 15 transports the reticle to the reticle stage 1, the reticle stage 18, or particle inspection unit 19.
An exposure apparatus, which uses an extreme ultraviolet light (EUV light), with a wavelength around 10 nm to 15 nm in the soft X-ray range or an electron beam (EB) is also under development as a next-generation light source. When the wavelength of exposure light decreases to the level of the EUV light or an electron beam, air under the atmospheric pressure no longer transmits the light. Hence, the optical path of the exposure light must be set to a high-vacuum environment of about 10−4 Pa to 10−5 PA or more. For this purpose, the reticle stage, including the projection lens end face and length measurement interference optical system, and the entire wafer stage must be arranged in a vacuum chamber, which is more hermetic than the F2 exposure apparatus. A load-lock chamber must be arranged at a wafer or reticle unloading/loading port. A wafer or reticle must be unloaded or loaded while keeping the vacuum degree in the vacuum chamber.
In EUV exposure, a material that transmits EUV light highly efficiently is not available. Hence, a reflection type mask having a pattern surface formed of a multilayered film is used, as disclosed in Japanese Patent Publication No. 7-27198. FIG. 4 is a schematic view of a reflection type mask used for EUV exposure. A reflection type mask 71 is made of a material having a coefficient of linear thermal expansion of 30 ppb/C.° or less. As the material of the reflection type mask 71, titanium-doped silica glass, a two-phase glass ceramic material, or the like, can be used. A multilayered film 72 is composed of a reflection layer having a multilayered film structure, such as Mo—Si, and an absorber, which absorbs soft X-rays, and forms an exposure pattern. A conductive film 73 is used to fix the reflection type mask 71 to an electrostatic chuck.
FIG. 3 is a view showing an arrangement of a mask stage in a semiconductor exposure apparatus, which uses EUV light as a light source. An EUV exposure stage is arranged above a reduction projection optical system in the exposure apparatus. A mask is held with its upper surface checked, so that a substrate is photosensitized by reflection light. Referring to FIG. 3, a reflection type mask stage 81 has an electrostatic mask 82, which holds the mask 71 by chucking with an electrostatic force during exposure. In a high vacuum atmosphere of about 10−4 Pa to 10−5 Pa, serving as an EUV exposure atmosphere, the mask cannot be held by conventional vacuum chucking, and a reticle is held by an electrostatic chuck using the Coulomb force or Johnson-Rahbeck force (to be referred to as the electrostatic force hereinafter), which is generally employed in a vacuum apparatus. In this case, in order to increase the force to hold the reticle, the conductive film 73 may be formed on the chuck holding surface on the mask 71, as disclosed in Japanese Patent Laid-Open No. 1-152727. Referring to FIG. 3, reference numeral 83 denotes a top plate; reference numeral 84, a linear motor movable element; and reference numeral 85, linear motor stators, respectively.
As described, when the wavelength of exposure light decreases to fall within the EUV range, no material can transmit the light efficiently, and no matter what existing material may be used, EUV light is absorbed undesirably. Accordingly, the conventional method of dust-proofing a reticle by a pellicle cannot be employed. In view of this, a removable pellicle is proposed, which is mounted on a reticle when transporting the reticle and removed from the reticle immediately before exposure.
If a removable pellicle is realized using a conventional pellicle, when a load-lock chamber is to be evacuated from the atmospheric pressure to a vacuum state (or vice versa), in order to move the reticle from the atmosphere into a vacuum, the pellicle may be broken by the pressure difference between the inside and outside of the pellicle. Hence, the pellicle must have a strength that can withstand such a pressure difference, or a new dustproof method must be proposed.
In order to prevent particles from entering the pellicle space, the pellicle must be fixed to the reticle such that the hermeticity of the pellicle space can be maintained, and must be detachably held so that it can be removed from the reticle easily for exposure. Japanese Patent Laid-Open No. 2002-252162 discloses holding the hermeticity using an O-ring. As the O-ring has adhesion, when it is brought into contact with the reticle, it adheres to the reticle. In this state, when the O-ring is separated from the reticle, dust is produced. The produced dust can attach to the pattern surface of the reticle with the electrostatic force. If the produced dust drops inside the removable pellicle, when the pellicle space is to be vacuum-evacuated or broken in the load-lock chamber, the produced dust may float in the pellicle space to undesirably attach to the pattern surface.
Management of only the particles on the pattern surface is not sufficient. When the reticle is fixed to the chuck on the exposure stage, if a particle is sandwiched between the chuck and reticle, it deforms the reticle to distort the pattern surface irregularly, leading to a decrease in exposure performance. It is said that in EUV exposure, with a degradation in accuracy (flatness) of the pattern surface of only about fifty nanometers, the exposure accuracy cannot be satisfied. Accordingly, the particle management on the nanometer order is necessary. In order to solve these problems, a method of decreasing the contact area of the chuck and reticle by using a pin chuck is available. With this method, although the probability of particle sandwiching can be decreased, it cannot be nullified.